Metal halide lamps each include a light-transmitting hermetic vessel having a pair of electrodes sealed therein, a discharge medium made of light emitting metal filled within the vessel. Examples of materials to be used for constituting light-transmitting hermetic vessels include quartz glass or light-transmitting ceramics. Then, light-transmitting hermetic vessels made of quartz glass are relatively inexpensive and further have higher in-line transmittance, so that such vessels are frequently and mainly used in sources for headlights, projection lights, and the like. In case of a light-transmitting hermetic vessel made of quartz glass, sealing portions joined to a surrounding portion are formed integrally with the surrounding portion so as to seal the surrounding portion having a discharge space formed therein.
To seal the light-transmitting hermetic vessel by the sealing portions, it is typical to hermetically embed metal foils inside the sealing portions, respectively. Then, the sealed metal foils each have one end at the surrounding portion side which end carries a proximal end of an electrode welded to the end, and other end at the opposite side which end carries an external lead member welded to the end, in a manner to supply current to the electrodes through the sealed metal foils, respectively.
Further, in case of the light-transmitting hermetic vessel made of quartz glass, the sealed metal foils embedded within the sealing portions and quartz glass surrounding the metal foils establish excellent hermetic junctions therebetween throughout a period of lighting of the metal halide lamp, thereby allowing to keep the interior of the light-transmitting hermetic vessel in an intended hermetic state and to supply current to the electrodes from the external lead members through the sealed metal foils, respectively. In case of formation of the sealing portions by utilizing the sealed metal foils, it is known to apply a satin crape treatment onto both obverse and reverse surfaces of the sealed metal foils by a sand blast method, an electrochemical method, or the like so as to increase a length of an outer periphery of each sealed metal foil by virtue of the thus formed fine irregularities on the surfaces, thereby restricting leaks which otherwise occur at the sealing portions (see patent-related reference 1).
There is also known a metal halide lamp adopting sealed metal foils to form sealing portions, respectively, such that each foil is formed with a plurality of holes penetrating through the foil in its thickness direction and dispersed over the surface of the foil, in a manner that the site of the foil formed with the holes has a cross section smaller than that of a site of the foil having no holes (see patent-related reference 2). It is stated in the patent-related reference 2 that the metal halide lamp described therein is improved in close contact property at the sealing portions, so that cracks and delaminations of the sealing portions are not caused, to restrict damages of the lamp over a lifetime of the lamp.
Meanwhile, there is known a so-called mercury-free metal halide lamp (hereinafter called “mercury-free lamp”, as expediency) having substantially no mercury filled therein (see patent-related reference 3). It is typical for a mercury-free lamp that the lamp includes, as a second halide sealed in the lamp, a halide of metal having a relatively high vapor pressure and insusceptible to emit light in a visible range such as zinc (Zn) halide, instead of mercury having been conventionally sealed in the lamp as a buffer substance for establishing a lamp voltage, and that a sealing pressure of rare gas for starting is set to be higher than that in case of a mercury-filling lamp in order to obtain an excellent rising-up characteristic of luminous flux.    Patent-related reference 1: Japanese Patent No. 3,150,918    Patent-related reference 2: Japanese Patent Application Laid-Open Publication No. 2001-266794    Patent-related reference 3: Japanese Patent Application Laid-Open Publication No. 11-238488